There has been continuous improvement in ethylene oxide catalysts in the past twenty years, particularly with respect to improvements in selectivity, activity and stability. Ethylene oxide/ethylene glycol (EO/EG) plants are usually designed for a particular ethylene oxide catalyst. If an improved catalyst utilizes a lower optimum operating temperature, the plant can not take the full advantage of the improvement, e.g., a catalyst with higher activity and lower optimum operating temperature may fail to generate enough medium or lower-pressure steam to meet the requirements for the operation of the plant. Steam is utilized as heating source for various equipment in EO/EG plants. When the operating temperature in the reactor is lower, the temperature of the coolant in the coolant condenser is also lower. The temperature difference (.DELTA.T) between the coolant and the steam generated in the steam generator drops. The surface area of the coolant condenser is fixed in a conventional EO/EG plant designed for a particular catalyst. Thus, the amount of medium-pressure steam generated would be reduced by a lower .DELTA.T.
To avoid the cost of adding a steam generator to make up the loss in steam production when operating with an improved catalyst which has a lower optimum operating temperature than the catalyst for which the plant was designed, the reactor would have to be operated at a temperature higher than the optimum operating temperature in order to raise the coolant temperature to one providing a sufficient .DELTA.T for generating enough medium-pressure steam. Since the reaction would not be operated at the optimum temperature of the catalyst, the selectivity decline rate would be accelerated. Such an accelerated decline could eventually cause a reduction in ethylene oxide production at later stages of catalyst life.
U.S. Pat. No. 4,074,660 issued Feb. 21, 1978, describes a process for cooling a high temperature reaction effluent to increase the heat recovery there from by cooling the effluent in a shell and tube heat exchanger having continuous tubes, with the shell being divided into two sections. In the first section, the effluent is cooled with water at a pressure that generates medium-pressure steam. In the second section, the effluent is cooled by water at a pressure that generates low-pressure steam or merely effects a preheating without vaporization. The system is described for use in waste-heat recovery from a high-temperature effluent such as the effluent from an ethyl benzene dehydrogenation reactor for the production of styrene. The temperature of that effluent is at least about 500.degree. F., and is generally in the order from about 600.degree. F. to about 2000.degree. F. That patent does not provide a solution to the problem of recovering heat from a coolant at a temperature below 500.degree. F. while generating sufficient steam in an existing conventional ethylene oxide plant with the existing coolant condensers.
EP No. 139,601, published Feb. 5, 1985 to Neel et al., describes a process for concentrating ethylene oxide solutions by steam distillation followed by multistage condensation in series connected heat exchangers. The so-described heat exchangers are designed for condensing the ethylene oxide solution rather than generating steam from the coolant for the ethylene oxide reactor.
A process has now been devised to recover residual heat in a conventional EO/EG plant designed for conventional catalyst(s) but utilizes improved catalyst(s) which also allows the plant to take optimum advantages of the improved activites and selectivities of the catalysts. The advantage of the present process is effected by the installation of auxiliary coolant condensing unit(s) to give a second, lower-pressure steam generation system and a change to a lighter coolant. Lower-pressure steam is additionally generated to be used in processes wherein a lower temperature would be adequate.